This application discloses a new method to record an image. It relates to microscopy and surface analysis in many scientific and technical fields. In particular it relates to the imaging and inspection of surfaces used in microelectronics: non-patterned and patterned wafers and photomasks. In the sense that it records electric amplitude, it relates to holography. Applications include microscopy, defect inspection, scatterometry, and optical metrology.
This work improves on interferometry, e.g. white-light interferometry, where successive interferograms with a phase-shift between them are used to construct an image which contains both phase and magnitude of the surface reflection coefficients (ref: patent by James Wyant). It also improves on ellipsometry where the relative magnitudes and phases between two states of polarization are measured, and the surface properties of a sample are deduced from an optical model of the surface (ref: patent by HDI, book by Azzam and BAshara). Finally it improves on direct-to-digital holography where the absolute phase and amplitude of light reflected or transmitted over an area is recorded in a single image. It also improves on scatterometry where the variation of reflection by polarization, angle of incidence, and/or wavelength is used to fit geometrical parameters in a model of microstructures, thus determining their size or shape.
In normal optical imaging, the intensity of the light is recorded. This intensity is the square of the magnitude of the electric field or amplitude:I(x,y)=|E(x,y)|2  (1)However, the electric field also has a phase that is lost in the detector. In the general case, it also has two polarizations P1 and P2 (or, sometimes labelled p and s,) which are typically linearly polarized with E-fields parallel to the x and y axises respectively, when xyz is an orthogonal coordinate system and z is the local direction of propagation. Thus, when only intensity is recorded, we have very little knowledge about the actual electric field in the image. We only know that:|Ep1(x,y)|2+|Ep2(x,y)|2=I(x,y)  (2)Images are often recorded in order to be used for analysis of the optical properties of a surface. Because only intensity information is recorded, the power of analysis is limited. This is the subject of so-called inverse imaging problems, where one or several images are combined with a priori knowledge to analyze surface characteristics beyond what the image alone shows. Generally, many objects can be inversely reconstructed from an intensity image, since they give the same intensity image, and one object must be chosen as more likely than another based on statistical properties or a priori knowledge. This has been the way most imaging and photography has worked since the dawn of photography and microscopy. If, however, more of the phase and amplitude information could be recorded in the image plane, a fuller analysis would be possible.
In a so-called phase-stepping interferometer, multiple interferograms are recorded with a known phase shift in the reference beam between the images. The set of images are used to calculate the variation in optical phase, i.e. the variation in optical path length, leading to a height map of a surface. In white-light interferometry (one variety of phase-stepping interferometry) a broad wavelength range is used in the interferometer, and the condition for exactly equal path length in test and reference beam can be established, thereby resolving the problem with multiple solutions in the interferometer. The white-light interferometer looks like a normal microscope with a camera. After analysis, the computer serving the white-light interferometer outputs one image similar to a normal microscope image and another image which is the optical phase or the height of the surface. The latter has a resolution of single-digit nanometers or better. In essence, phase-stepping and white-light interferometers record the magnitude and phase of the electric field from a series of images. The phase-stepping and white-light interferometers are good for accurate height metrology.
Another way to capture more information is by ellipsometry. In ellipsometiy a light beam is reflected onto the surface, normally at a high angle of incidence. The incident beam is polarized in a known way, typically having linear polarization with the polarization direction at 45 degrees to the plane of incidence. The surface affects light polarized parallel (“p”) and perpendicular (“s”) to the plane of incidence differently. One can imagine that at the point of reflection the beam is split into a p and an s beam. They are reflected with different attenuation and phase delay and recombined instantly to give a polarization that is different from the incident polarization. By measuring the polarization before and after and comparing the two, the difference in amplitude and phase between the p and s beams can be determined. By comparison to an optical model of the surface, two selected parameters of the surface (often thickness and refractive index of a surface film) can be determined. Ellipsometry can also produce images by the combination of the polarizing system with imaging optics. Because it uses the difference between two components of the same light beam as the information-carrying quantity, ellipsometry has low noise and is extremely sensitive to small surface changes.
A third background technology is so-called direct-to-digital holography, or DDH. The image from a microscope is superposed on the image detector by a reference blanket illumination beam, coherent with the light from the image. The reference beam has an angular offset from the light from the image and a dense fringe pattern is produced on the sensor. The contrast of the fringes gives the magnitude of the E-field in the image, and the fringe placement gives the phase. Thus an image with both magnitude and phase can be calculated as in the phase-stepping interferometer, but from a single recorded image. DDH has the benefit that it is fast. With a single exposure needed at each geometric position, DDH is suitable for scanning large surfaces for defects. The defects found may not be visible in a normal microscope image. DDH can therefore be used as a complement to bright- and dark-field images for defect inspection, especially for defects that extend in the z direction.
All these three background techniques extract information about the E-field reflected or transmitted by the sample; however they extract just part of the full information. Ellipsometry sees only the difference between the polarizations; phase-stepping and DDH see only an average phase and magnitude of the two polarizations.
An opportunity arises to collect more information at once, in a single instrument. Better and more complete optical analysis components and systems may result.